In some physics textbooks and articles, certain quantities are defined with the opposite sign from that which is used in other publications. This lack of standardization is a frequent source of confusion, misunderstandings, and even outright errors. A standard choice of sign for a definition is called a sign convention. Sometimes, this term is used more broadly to include factors of i and 2π.

A noncomprehensive list of sign conventions:

• In relativity, the metric signature could either be +--- or -+++.
• The choice of in the Dirac equation.
• The sign of the Faraday 2-form in gauge theories.
• The sign of work in the first law of thermodynamics.
• Time dependence of an electromagnetic wave:
  • exp( - iωt) (mainly used by physicists)
  • exp( + jωt) (mainly used by engineers)

It is often considered good form to state explicitly which sign convention is to be used at the beginning of each book or article.